1. Field of the Invention
The invention pertains to the measurement of surface or interfacial tension between two fluid phases, gas and liquid or liquid and liquid. This basic physical property of fluid interfaces is important to most fields of technological art in which gas-liquid and liquid-liquid interfaces or menisci occur, including but not limited to detergency, solubilization, microemulsification, emulsification, demulsification, foaming, defoaming, aeration, absorption, distillation, extraction, metallurgical and polymeric melt processing, crystal growing, various fluid phase reaction methods, and formulations of wetting agents, spreading agents, lubricants, drilling fluids for well-drilling, adhesives, paints, coatings, photographic films, chemical solutions for enhanced oil recovery, etc.
The invention pertains to the measurement of all magnitudes of interfacial tension but is especially advantageous for measurement of tensions less than 1 dyn/cm (10.sup.-3 newton/meter) and is even more advantageous for measurement of tensions less than 10.sup.-2 dyn/cm (10.sup.-5 newton/meter). It is also advantageous for measurements at temperatures substantially different from ambient.
2. Description of the Prior Art
Conventional methods of measuring fluid interfacial tensions are the force methods, the shape methods, and several miscellaneous methods. The principal force methods are the well-known techniques of the Du Nouy ring and the Wilhelmy plate. These methods are generally not suitable for tensions less than 1 dyn/cm, because the force becomes too small to be accurately measured. The principal shape methods are the sessile drop and the pendent drop, both well-suited for low interfacial tension measurements. Other miscellaneous methods include the differential capillary rise, drop weight, maximum bubble pressure, and light scattering techniques. Though capable of measuring low tensions, they are not widely used owing not only to the tedious measurement procedure involved, but also to other requirements and limitations. The state of art is summarized by A. W. Adamson, Physical Chemistry of Surfaces, 3rd edition, Wiley (1976).
The spinning drop technique is a shape method but is not conventional. The method, first devised by Vonnegut in 1942 (B. Vonnegut, Rev. Sci. Instr. 11 6 (1942)), has received its development in connection with low interfacial tensions:
Silberberg (A. Silberberg, Ph.D. Thesis, University of Basel, Switzerland, 1952) measured low tensions in a polymer system by means of the spinning drop method. He noted the effects of bearing heating and the consequences on the tension.
Princen et al (H. M. Princen, I.Y.Z. Zia and S. G. Mason, J. Colloid Interface Sci. 23, 99 (1967)) solved the spinning drop shape problem by means of elliptic integrals and showed how to solve explicitly for the tension. They developed a variable speed spinning drop apparatus for measuring surface and interfacial tensions. But instead of measuring drop diameter they chose to measure drop length and volume and from these to calculate tension. The reason evidently is that this approach avoids the need to determine the cylindrical lens effect of the sample tube. It depends on the drop volume remaining constant at the volume injected, which is often violated.
Ryden and Albertsson (J. Ryden and P. Albertsson, J. Colloid Interface Sci. 37, 219 (1971)), using essentially the method of Princen et al, measured ultralow tensions (&lt;0.01 dyn/cm) in a phase-separated polymer system. Their operating speed range of 200 to 450 rpm may sometimes be suitable for viscous polymer systems but is too low to achieve needed gyrostatic equilibrium in less viscous systems.
Patterson et al (H. T. Patterson, K. H. Hu and T. H. Grindstaff, J. Polymer Sci. Part C, 34, 31 (1971)) modified the method for measuring tensions of molten polymer solutions. They measured the drop diameter photographically and used experimentally-determined magnification factors in their calculation of tensions from measurements.
Torza (S. Torza, Rev. Sci. Instru. 46, 778 (1975)) designed a spinning drop apparatus using air bearings to avoid bearing heating and a magnetic coupling to reduce vibration. The instrument is relatively vibration free up to 7,000 rpm, but not beyond. It employed a comparatively large diameter of sample tube.
Of the spinning drop instruments described, all but Silberberg's and Ryden and Albertsson's were designed for measuring relatively high tensions. Cayias et al (J. L. Cayias, R. S. Schechter and W. H. Wade, in Adsorption at Interfaces (edited by K. L. Mittal), ACS Symposium Series 8, pp. 234-247 (1975)) much improved the method in an attempt to accommodate ultralow interfacial tensions. Their design, known as the University of Texas Spinning Drop Interfacial Tensiometer, has become widely used for low tension measurements in the petroleum recovery art. The design is also copied and attempts have been made to improve upon it.
Gash and Parrish (B. Gash and D. R. Parrish, J. Pet. Tech. 29, 30 (1977)) developed a constant-speed spinning drop apparatus for surfactant screening. They mounted the sample tube directly to the motor shaft by an aluminum collar, which avoided the use of bearings. The relatively low operating speed of 3,600 rpm is probably not sufficiently high to ensure gyrostatic equilibrium, and vibration may be serious.
The evolution of the spinning drop method shows that no previous investigators correctly recognized the nature of departures from gyrostatic equilibrium which are brought about by vibrations, eccentricity, varying speed, bearing heating and temperature variations. Only Torza tried air bearings but he did not consider the matter of coaxiality. In his measurements he avoided drop diameter and relied on constancy of drop volume, which is not uncommonly inconstant, especially in low tensions systems.
No previously described instrument has adequate provisions for establishing that gyrostatic equilibrium is approached closely enough by the contents of the sample tube to permit reliable measurements of interfacial tension.
No previously described instrument has been designed on the basis of hydrodynamic principles and an analysis of all of the expected errors. Shortcomings of previously described instruments include bearing heating and non-isothermal sample holder, lack of coaxiality of the bore of the sample holder and the rotation axis, vibrations from bearings and motor, limited visual access to the sample holder, absence of provision for sample holders of different inside diameters, insufficiently high rotational speeds, and inadequate provision for leveling the axis of rotation.